Hitherto, golf balls are typically classified into thread wound golf balls and solid golf balls. The solid golf ball, particularly two-piece solid golf ball is generally approved or employed by most of amateur golfers, because it has better durability and better flight performance than the thread wound golf ball. It is, however, generally known that the two-piece golf ball exhibits better durability and better flight performance, but is poor in shot feel.
It has been attempted to make soft the shot feel of the two-piece solid golf ball equal to thread wound golf balls by adjusting a hardness of a core or cover. The improvement of the two-piece solid golf ball for making shot feel soft has also been conducted by making its core or cover two or more layers. The golf ball whose core or cover is made two or more layers is called as a multi-piece solid golf ball.
Among the multi-piece solid golf balls, a three-piece solid golf ball having two-layered core or two-layered cover has been mostly developed. In the three-piece solid golf ball, a two-layered core-type three-piece golf ball, of which the core is made two-layered, has been usefully employed. One embodiment of the method of producing the two-layered core-type three-piece golf ball will be explained.
A center for the two-layered core is vulcanized or press-molded, and an intermediate layer is covered on the center and vulcanized or press-molded to obtain a two-layered core. The resulting core is covered with a cover to obtain a three-piece solid golf ball. The step of producing the two-layered core will be explained in detail. The center is obtained by mixing a rubber composition which is formulated for obtaining desired physical properties to form plugs having a proper size, putting the plugs in a mold, and vulcanizing or press-molding the rubber composition in a spherical shape. The intermediate layer is obtained by mixing a rubber composition for the intermediate layer to form plugs, putting the plugs in two molds having a semi-spherical cavity, pressing the rubber composition using a male plug mold to form a semi-spherical concave, inserting the center in one concave, putting the other concave on it to cover on the center, and vulcanizing or press-molding it to form a two-layered core.
In the step of covering the center with two of the rubber compositions for the intermediate layer, since the rubber compositions are unvulcanized, two of the rubber compositions are adhered with each other for a while. However, the two rubber compositions are separated from the adhesive surface because of the shrinking characteristics of the rubber composition. When vulcanizing or press-molding the rubber composition with separation of the adhesive surface, the portions of the intermediate layer with separation of the adhesive surface and the opposite portion of the intermediate layer to the center have different thickness of the intermediate layer, and thus the center is eccentric. The eccentricity of the center gives unevenness of physical properties of the resulting golf ball, and thus it is required to restrain the eccentricity as low as possible.